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According to the NBD spec, a server that advertises NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will not see any cache inconsistencies: when properly separated by a single flush, actions performed by one client will be visible to another client, regardless of which client did the flush. We always satisfy these conditions in qemu - even when we support multiple clients, ALL clients go through a single point of reference into the block layer, with no local caching. The effect of one client is instantly visible to the next client. Even if our backend were a network device, we argue that any multi-path caching effects that would cause inconsistencies in back-to-back actions not seeing the effect of previous actions would be a bug in that backend, and not the fault of caching in qemu. As such, it is safe to unconditionally advertise CAN_MULTI_CONN for any qemu NBD server situation that supports parallel clients. Note, however, that we don't want to advertise CAN_MULTI_CONN when we know that a second client cannot connect (for historical reasons, qemu-nbd defaults to a single connection while nbd-server-add and QMP commands default to unlimited connections; but we already have existing means to let either style of NBD server creation alter those defaults). This is visible by no longer advertising MULTI_CONN for 'qemu-nbd -r' without -e, as in the iotest nbd-qemu-allocation. The harder part of this patch is setting up an iotest to demonstrate behavior of multiple NBD clients to a single server. It might be possible with parallel qemu-io processes, but I found it easier to do in python with the help of libnbd, and help from Nir and Vladimir in writing the test. Signed-off-by: Eric Blake <eblake@redhat.com> Suggested-by: Nir Soffer <nsoffer@redhat.com> Suggested-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru> Message-Id: <20220512004924.417153-3-eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
410 lines
12 KiB
Python
410 lines
12 KiB
Python
# -*- Mode: Python -*-
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# vim: filetype=python
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##
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# == Block device exports
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##
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{ 'include': 'sockets.json' }
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{ 'include': 'block-core.json' }
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##
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# @NbdServerOptions:
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#
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# Keep this type consistent with the nbd-server-start arguments. The only
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# intended difference is using SocketAddress instead of SocketAddressLegacy.
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#
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# @addr: Address on which to listen.
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# @tls-creds: ID of the TLS credentials object (since 2.6).
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# @tls-authz: ID of the QAuthZ authorization object used to validate
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# the client's x509 distinguished name. This object is
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# is only resolved at time of use, so can be deleted and
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# recreated on the fly while the NBD server is active.
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# If missing, it will default to denying access (since 4.0).
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# @max-connections: The maximum number of connections to allow at the same
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# time, 0 for unlimited. Setting this to 1 also stops
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# the server from advertising multiple client support
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# (since 5.2; default: 0)
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#
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# Since: 4.2
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##
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{ 'struct': 'NbdServerOptions',
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'data': { 'addr': 'SocketAddress',
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'*tls-creds': 'str',
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'*tls-authz': 'str',
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'*max-connections': 'uint32' } }
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##
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# @nbd-server-start:
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#
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# Start an NBD server listening on the given host and port. Block
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# devices can then be exported using @nbd-server-add. The NBD
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# server will present them as named exports; for example, another
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# QEMU instance could refer to them as "nbd:HOST:PORT:exportname=NAME".
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#
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# Keep this type consistent with the NbdServerOptions type. The only intended
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# difference is using SocketAddressLegacy instead of SocketAddress.
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#
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# @addr: Address on which to listen.
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# @tls-creds: ID of the TLS credentials object (since 2.6).
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# @tls-authz: ID of the QAuthZ authorization object used to validate
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# the client's x509 distinguished name. This object is
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# is only resolved at time of use, so can be deleted and
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# recreated on the fly while the NBD server is active.
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# If missing, it will default to denying access (since 4.0).
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# @max-connections: The maximum number of connections to allow at the same
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# time, 0 for unlimited. Setting this to 1 also stops
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# the server from advertising multiple client support
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# (since 5.2; default: 0).
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#
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# Returns: error if the server is already running.
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#
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# Since: 1.3
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##
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{ 'command': 'nbd-server-start',
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'data': { 'addr': 'SocketAddressLegacy',
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'*tls-creds': 'str',
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'*tls-authz': 'str',
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'*max-connections': 'uint32' } }
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##
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# @BlockExportOptionsNbdBase:
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#
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# An NBD block export (common options shared between nbd-server-add and
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# the NBD branch of block-export-add).
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#
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# @name: Export name. If unspecified, the @device parameter is used as the
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# export name. (Since 2.12)
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#
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# @description: Free-form description of the export, up to 4096 bytes.
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# (Since 5.0)
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#
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# Since: 5.0
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##
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{ 'struct': 'BlockExportOptionsNbdBase',
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'data': { '*name': 'str', '*description': 'str' } }
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##
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# @BlockExportOptionsNbd:
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#
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# An NBD block export (distinct options used in the NBD branch of
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# block-export-add).
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#
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# @bitmaps: Also export each of the named dirty bitmaps reachable from
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# @device, so the NBD client can use NBD_OPT_SET_META_CONTEXT with
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# the metadata context name "qemu:dirty-bitmap:BITMAP" to inspect
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# each bitmap.
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# Since 7.1 bitmap may be specified by node/name pair.
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#
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# @allocation-depth: Also export the allocation depth map for @device, so
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# the NBD client can use NBD_OPT_SET_META_CONTEXT with
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# the metadata context name "qemu:allocation-depth" to
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# inspect allocation details. (since 5.2)
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#
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# Since: 5.2
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##
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{ 'struct': 'BlockExportOptionsNbd',
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'base': 'BlockExportOptionsNbdBase',
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'data': { '*bitmaps': ['BlockDirtyBitmapOrStr'],
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'*allocation-depth': 'bool' } }
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##
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# @BlockExportOptionsVhostUserBlk:
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#
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# A vhost-user-blk block export.
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#
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# @addr: The vhost-user socket on which to listen. Both 'unix' and 'fd'
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# SocketAddress types are supported. Passed fds must be UNIX domain
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# sockets.
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# @logical-block-size: Logical block size in bytes. Defaults to 512 bytes.
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# @num-queues: Number of request virtqueues. Must be greater than 0. Defaults
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# to 1.
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#
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# Since: 5.2
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##
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{ 'struct': 'BlockExportOptionsVhostUserBlk',
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'data': { 'addr': 'SocketAddress',
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'*logical-block-size': 'size',
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'*num-queues': 'uint16'} }
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##
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# @FuseExportAllowOther:
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#
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# Possible allow_other modes for FUSE exports.
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#
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# @off: Do not pass allow_other as a mount option.
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#
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# @on: Pass allow_other as a mount option.
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#
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# @auto: Try mounting with allow_other first, and if that fails, retry
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# without allow_other.
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#
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# Since: 6.1
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##
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{ 'enum': 'FuseExportAllowOther',
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'data': ['off', 'on', 'auto'] }
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##
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# @BlockExportOptionsFuse:
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#
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# Options for exporting a block graph node on some (file) mountpoint
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# as a raw image.
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#
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# @mountpoint: Path on which to export the block device via FUSE.
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# This must point to an existing regular file.
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#
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# @growable: Whether writes beyond the EOF should grow the block node
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# accordingly. (default: false)
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#
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# @allow-other: If this is off, only qemu's user is allowed access to
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# this export. That cannot be changed even with chmod or
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# chown.
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# Enabling this option will allow other users access to
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# the export with the FUSE mount option "allow_other".
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# Note that using allow_other as a non-root user requires
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# user_allow_other to be enabled in the global fuse.conf
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# configuration file.
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# In auto mode (the default), the FUSE export driver will
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# first attempt to mount the export with allow_other, and
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# if that fails, try again without.
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# (since 6.1; default: auto)
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#
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# Since: 6.0
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##
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{ 'struct': 'BlockExportOptionsFuse',
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'data': { 'mountpoint': 'str',
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'*growable': 'bool',
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'*allow-other': 'FuseExportAllowOther' },
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'if': 'CONFIG_FUSE' }
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##
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# @NbdServerAddOptions:
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#
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# An NBD block export, per legacy nbd-server-add command.
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#
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# @device: The device name or node name of the node to be exported
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#
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# @writable: Whether clients should be able to write to the device via the
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# NBD connection (default false).
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#
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# @bitmap: Also export a single dirty bitmap reachable from @device, so the
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# NBD client can use NBD_OPT_SET_META_CONTEXT with the metadata
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# context name "qemu:dirty-bitmap:BITMAP" to inspect the bitmap
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# (since 4.0).
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#
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# Since: 5.0
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##
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{ 'struct': 'NbdServerAddOptions',
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'base': 'BlockExportOptionsNbdBase',
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'data': { 'device': 'str',
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'*writable': 'bool', '*bitmap': 'str' } }
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##
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# @nbd-server-add:
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#
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# Export a block node to QEMU's embedded NBD server.
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#
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# The export name will be used as the id for the resulting block export.
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#
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# Features:
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# @deprecated: This command is deprecated. Use @block-export-add instead.
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#
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# Returns: error if the server is not running, or export with the same name
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# already exists.
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#
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# Since: 1.3
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##
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{ 'command': 'nbd-server-add',
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'data': 'NbdServerAddOptions', 'boxed': true, 'features': ['deprecated'] }
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##
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# @BlockExportRemoveMode:
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#
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# Mode for removing a block export.
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#
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# @safe: Remove export if there are no existing connections, fail otherwise.
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#
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# @hard: Drop all connections immediately and remove export.
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#
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# TODO: Potential additional modes to be added in the future:
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#
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# hide: Just hide export from new clients, leave existing connections as is.
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# Remove export after all clients are disconnected.
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#
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# soft: Hide export from new clients, answer with ESHUTDOWN for all further
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# requests from existing clients.
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#
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# Since: 2.12
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##
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{'enum': 'BlockExportRemoveMode', 'data': ['safe', 'hard']}
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##
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# @nbd-server-remove:
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#
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# Remove NBD export by name.
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#
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# @name: Block export id.
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#
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# @mode: Mode of command operation. See @BlockExportRemoveMode description.
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# Default is 'safe'.
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#
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# Features:
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# @deprecated: This command is deprecated. Use @block-export-del instead.
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#
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# Returns: error if
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# - the server is not running
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# - export is not found
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# - mode is 'safe' and there are existing connections
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#
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# Since: 2.12
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##
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{ 'command': 'nbd-server-remove',
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'data': {'name': 'str', '*mode': 'BlockExportRemoveMode'},
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'features': ['deprecated'] }
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##
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# @nbd-server-stop:
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#
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# Stop QEMU's embedded NBD server, and unregister all devices previously
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# added via @nbd-server-add.
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#
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# Since: 1.3
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##
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{ 'command': 'nbd-server-stop' }
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##
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# @BlockExportType:
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#
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# An enumeration of block export types
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#
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# @nbd: NBD export
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# @vhost-user-blk: vhost-user-blk export (since 5.2)
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# @fuse: FUSE export (since: 6.0)
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#
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# Since: 4.2
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##
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{ 'enum': 'BlockExportType',
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'data': [ 'nbd',
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{ 'name': 'vhost-user-blk',
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'if': 'CONFIG_VHOST_USER_BLK_SERVER' },
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{ 'name': 'fuse', 'if': 'CONFIG_FUSE' } ] }
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##
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# @BlockExportOptions:
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#
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# Describes a block export, i.e. how single node should be exported on an
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# external interface.
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#
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# @id: A unique identifier for the block export (across all export types)
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#
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# @node-name: The node name of the block node to be exported (since: 5.2)
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#
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# @writable: True if clients should be able to write to the export
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# (default false)
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#
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# @writethrough: If true, caches are flushed after every write request to the
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# export before completion is signalled. (since: 5.2;
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# default: false)
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#
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# @iothread: The name of the iothread object where the export will run. The
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# default is to use the thread currently associated with the
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# block node. (since: 5.2)
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#
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# @fixed-iothread: True prevents the block node from being moved to another
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# thread while the export is active. If true and @iothread is
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# given, export creation fails if the block node cannot be
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# moved to the iothread. The default is false. (since: 5.2)
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#
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# Since: 4.2
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##
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{ 'union': 'BlockExportOptions',
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'base': { 'type': 'BlockExportType',
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'id': 'str',
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'*fixed-iothread': 'bool',
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'*iothread': 'str',
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'node-name': 'str',
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'*writable': 'bool',
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'*writethrough': 'bool' },
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'discriminator': 'type',
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'data': {
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'nbd': 'BlockExportOptionsNbd',
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'vhost-user-blk': { 'type': 'BlockExportOptionsVhostUserBlk',
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'if': 'CONFIG_VHOST_USER_BLK_SERVER' },
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'fuse': { 'type': 'BlockExportOptionsFuse',
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'if': 'CONFIG_FUSE' }
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} }
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##
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# @block-export-add:
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#
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# Creates a new block export.
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#
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# Since: 5.2
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##
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{ 'command': 'block-export-add',
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'data': 'BlockExportOptions', 'boxed': true }
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##
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# @block-export-del:
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#
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# Request to remove a block export. This drops the user's reference to the
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# export, but the export may still stay around after this command returns until
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# the shutdown of the export has completed.
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#
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# @id: Block export id.
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#
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# @mode: Mode of command operation. See @BlockExportRemoveMode description.
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# Default is 'safe'.
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#
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# Returns: Error if the export is not found or @mode is 'safe' and the export
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# is still in use (e.g. by existing client connections)
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#
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# Since: 5.2
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##
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{ 'command': 'block-export-del',
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'data': { 'id': 'str', '*mode': 'BlockExportRemoveMode' } }
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##
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# @BLOCK_EXPORT_DELETED:
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#
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# Emitted when a block export is removed and its id can be reused.
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#
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# @id: Block export id.
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#
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# Since: 5.2
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##
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{ 'event': 'BLOCK_EXPORT_DELETED',
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'data': { 'id': 'str' } }
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##
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# @BlockExportInfo:
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#
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# Information about a single block export.
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#
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# @id: The unique identifier for the block export
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#
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# @type: The block export type
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#
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# @node-name: The node name of the block node that is exported
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#
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# @shutting-down: True if the export is shutting down (e.g. after a
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# block-export-del command, but before the shutdown has
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# completed)
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#
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# Since: 5.2
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##
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{ 'struct': 'BlockExportInfo',
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'data': { 'id': 'str',
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'type': 'BlockExportType',
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'node-name': 'str',
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'shutting-down': 'bool' } }
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##
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# @query-block-exports:
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#
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# Returns: A list of BlockExportInfo describing all block exports
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#
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# Since: 5.2
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##
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{ 'command': 'query-block-exports', 'returns': ['BlockExportInfo'] }
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